Valve regulated lead-acid batteries (VRLA) are used in telecommunications and other applications to provide an energy reserve to equipment for operation when AC power is unavailable. VRLA batteries are usually connected to the output of one or more AC/DC rectifiers. The DC voltage generated by the rectifier is then used to power the equipment, commonly referred to as the load. This component configuration allows VRLA batteries to instantly provide power to the load in the event of AC failure or when one or more rectifiers becomes unavailable. VRLA batteries are usually maintained in a charged state by maintaining the rectifier voltage at a constant level at a particular temperature. Battery charging current flows until the battery is fully charged, and the battery's voltage approaches the rectifier voltage. After charging, current diminishes to a decreasing level, referred to as the float current, that is adequate to maintain the battery in a fully charged state.
One of the deficiencies of the float charging approach is that when a average voltage, for example, of 2.25 v/cell to 2.27 v/cell is applied between the positive and negative terminals of the battery, this voltage is not appropriately shared between the terminals of the battery because of the oxygen recombination reaction. The oxygen recombination reaction under normal float conditions is a spontaneous reaction which depolarizes the negative electrode of the battery. Since the battery charging is done by applying a constant voltage across the battery terminals, the depolarization on the negative terminal must be balanced by a corresponding increase in the positive electrode polarization. This imbalance causes the depolarized negative electrode to go into a self-discharge mode, which causes the battery to lose capacity with time and results in premature failure of the battery.
One method of improving battery charging is to utilize a battery having a reference electrode and a matched charging circuit.